我遇到的问题是pyspark无法加载 com.datastax.spark:spark-cassandra-connector_2.11:2.5.1 当单元测试（或pytesting）时。当在普通代码中使用包时，它会很好地加载库，但在unittest中，永远不会加载库，并且会发生错误。
错误：

java.lang.ClassNotFoundException: Failed to find data source: org.apache.spark.sql.cassandra. Please find packages at http://spark.apache.org/third-party-projects.html

代码段：

conf = SparkConf()
    conf.setAppName(XXX)
    conf.setMaster(XXX)
    conf.set("spark.jars.packages", "com.datastax.spark:spark-cassandra-connector_2.11:2.5.1")

    sc = SparkContext(conf=conf)
    spark = SparkSession(sc)

    df = spark.read.format("org.apache.spark.sql.cassandra") \
        .option("keyspace", "system") \
        .option("table", "local") \
        .option("spark.cassandra.auth.username", XXX) \
        .option("spark.cassandra.auth.password", XXX) \
        .option("spark.cassandra.connection.host", XXX) \
        .load()

有什么解决办法吗？Cassandra？